1. Technical Field
An embodiment of the present invention generally relates to an ultrasonic flow sensor. More particularly, an embodiment of the present invention relates to a transit-time ultrasonic flow sensor with a parabolic reflecting surface to measure a flow rate.
2. Discussion of the Related Art
The use of the insertion monitoring devices to measure fluid flow rate are restricted by cost and practical problems. For example, in one known insertion metering device, a probe is inserted into a duct through a hole or valve opening in the duct wall. The probe comprises a rod which carries a turbine or electromagnetic sensing element on its tip. The sensing element can take a point measurement indicative of the flow in a part of the duct at a point in time. However, because the flow in the duct is unknown, (varying both in profile across the cross-section of the duct and with time) several measurements must be taken at different points in the cross-section of the duct and at different times. An average can then be built up which would approximate the average flow rate. Its accuracy is limited by the difficulty in aligning the sensing element correctly along the axis of the duct.
In order to obtain reasonably accurate results, the prior art insertion technique requires that measurements be taken at several positions across at least one diameter of the duct. However, it has been found that in practice where flow profiles are distorted, it is necessary to measure across more than one diameter (i.e. two orthogonal diameters) to provide sufficiently accurate results which can be used for calibration. This introduces severe problems when the duct system is installed underground, as it requires that a large chamber must be excavated around the duct in order to allow access for separate circumferentially spaced holes in the duct to be made to allow the orthogonal measurements to be made. Additional problems may be encountered with duct systems installed in aircraft where access may also be difficult.
A further problem with the prior art technique is that the surface area of the rod which supports the sensing element forms a variable blockage in the duct as the element is moved across the diameter. This blockage affects the results by altering the flow profile in the duct and increases turbulence. Furthermore, the process of taking the many measurements required is subject to variability due to the often difficult operating conditions in which the measurements must be made. For example, the insertion probe operator may be working in a water filled, muddy pit which makes it difficult to obtain the various readings with any certain degree of accuracy.
Several different sensor configurations have also been used including: 1) direct measurement of a propagation time of a pulse emitted by a first transducer and received by a second transducer, where the change in time is a function of fluid flow rate; 2) dual “sing-around” sound velocimeters, where the difference in “sing-around” frequency between the velocimeters is a function of the fluid flow rate; 3) sensors producing continuous waves using two widely different high frequency carriers but commonly modulated with another much lower frequency signal, where the phase difference of the modulated signal on the received carriers is a function of the fluid flow rate; and 4) sensors producing bursts of continuous waves, using a single frequency on a pair of transducers, the burst duration being less than the acoustic propagation time between the transducers, where the time between the received transmissions is a function of flow rate.
Transit-time ultrasonic flow sensors, also known as “time-of-flight” ultrasonic flow sensors, detect the acoustic propagation time difference between upstream and downstream ultrasonic transmissions, resulting from movement of flowing fluid, and process this information to derive a fluid flow rate.
Transducers of transit-time ultrasonic flow sensors are most often field mounted. They are generally individually attached to the outside of a pipe. Unlike other types of ultrasonic flow sensors, such as Doppler ultrasonic flow sensors, transit-time ultrasonic flow sensors typically do not require placing a transducer inside a pipe in order to make a flow measurement. However, measurement accuracy may be compromised by a multitude of factors, such as pipe wall integrity, pipe surface condition, and distance between transducers.
Even when the transducers are in contact with the fluid being measured (wetted), the transducers may become misaligned, i.e., disposed at the wrong distance or angle, resulting in measurement error. Thus, sensors having wetted transducers are typically equipped with supporting electronics that include sophisticated diagnostics for confirming proper installation and operation. Consequently, such sensors are relatively expensive and have a reputation for occasionally producing erroneous measurements.